Typically, in order for an internal combustion engine to aspirate intake air into a cylinder in its intake cycle and to exhaust or discharge combustion gases out of the cylinder in its exhaust cycle, the intake and exhaust valves are timed to be open simultaneously during an overlap period. It is well known in the art that the valve overlap period affects the intake and exhaust performance of the engine and, therefore, the performance of the engine.
In order for the internal combustion engine to have stable fuel combustion, it is necessary for the valve overlap period to be short in time, or small in degrees, during idling. In other words, because, during idling, the amount of fuel mixture supplied is small, the stability of fuel combustion is relatively poor, and a boost negative pressure is high, combustion gases are easily aspirated into, and retained so that they reside in, the combustion chamber of the cylinder. For this reason, if the valve overlap is long in time, or large in degrees, the stability of fuel combustion is considerably worsened.
On the other hand, it has been thought that under higher or heavier engine loads, engine output torque can be raised by setting the valve overlap so that it is long or large. This is because a long or large valve overlap develops a high charging efficiency with inertia effects in intake air.
Recently, as is described in, for instance, Japanese Unexamined Patent Publication No. 62 - 191636, it has been proposed to install a valve timing changing mechanism for intake valves, exhaust valves, or both in an internal combustion engine. The valve timing changing mechanism changes valve timings so as to shorten or reduce the valve overlap during idling and to prolong or increase it while the engine operates under engine operating conditions other than engine idling. The valve timing changing mechanism described in the above publication sets the valve overlap so that it is generally longer or larger under higher or heavier engine loads in a low and a high speed range, but shorter or smaller in a specific high speed range in order to improve engine output torque.
The effects of valve overlap have been thoroughly considered in an attempt to ensure the idling stability of the engine and to improve the charging efficiency of intake air and the output torque of the engine. However, no sufficient consideration has been made in connection with the effects of valve overlap on engine performance, and there has not been any attempt to vary the valve overlap period in accordance with engine operating conditions, such as, in particular, engine speed and engine load, in order for the engine to reduce harmful emissions containing oxides of nitrogen (NOx) and hydrocarbon (HC) and to improve fuel economy.
Considering the effect of valve overlap on engine performance, it is desirable for the valve overlap of the intake and exhaust valves to increase so that it is longer in time, or larger in degrees, as the engine speed, usually in rpm, increases, in order to increase the output torque of the engine for full-throttle engine operating conditions.
In connection with oxides of nitrogen (NOx), however, the residual gases (including a reflux of exhaust gas from the exhaust system) in the combustion chamber will increase with an increase in valve overlap. This is due to what is known as "internal exhaust gas recirculation" or internal EGR. This internal exhaust gas recirculation, or internal EGR, is significant, particularly when the negative boost pressure is high while the engine operates at lower or lighter engine loads. The internal exhaust gas recirculation (internal EGR) causes a drop in fuel combustion stability and a reduction in the content of oxides of nitrogen (NOx) of exhaust gases.
Regarding hydrocarbon (HC), when an engine load does not change, the hydrocarbon (HC) content of exhaust gas decreases with a reduction in valve overlap for lower engine speeds and with an increase in valve overlap for higher engine speeds. Further, when engine speed does not change, .the hydrocarbon (HC) content of exhaust gas decreases with a reduction in valve overlap for lower engine loads and with an increase in valve overlap for higher engine loads. As far as the hydrocarbon (HC) content of exhaust gas is concerned, it is preferable to increase the valve overlap with an increase in engine speed or with an increase in engine load.
Pumping loss for intake air also decreases with an increase in residual gas in the combustion chamber, and fuel efficiency is improved with a decrease in the hydrocarbon (HC) content of exhaust gas. External exhaust gas recirculation (EGR), which is utilized to reduce the content of oxides of nitrogen (NOx) of exhaust gas, does not always cause a reflux of unburned gas, which is produced when the fuel adheres to the cylinder wall during a later stage of an exhaust cycle and is high in concentration, into the combustion chamber. Therefore, no reduction in hydrocarbon (HC) content is induced. Furthermore, since the temperature of recirculating exhaust gas is low, the pumping loss does not sufficiently decrease.
Increasing the valve overlap period or angle under lower or lighter engine loads is undesirable with respect to the stability of fuel combustion and idling stability. Further, increasing the valve overlap period or angle under higher or heavier engine loads is, although desirable for oxides of nitrogen (NOx) and hydrocarbon (HC) in order to reduce its content, undesirable with respect to the output torque of engine. This is because when increasing the valve overlap under higher or heavier engine loads, the pressure of exhaust gas becomes higher, resulting in making it difficult to easily aspirate or charge fresh air and, accordingly, lowering the efficiency of charging fresh air, so that the output torque of the engine is dropped adversely.
The valve timing changing mechanism described in the above mentioned publication, as was previously described, can increasingly change a valve operation timing to vary the valve overlap, so that it is long in time or large in degrees under higher or heavier engine loads in a medium range of engine speeds. The mechanism also can decreasingly change the timing to vary the valve overlap so that it is short or small under medium engine loads. Although desirable to reduce the content of oxides of nitrogen (NOx) and hydrocarbon (HC), changing the valve overlap time in such a way is disadvantageous in many aspects. For example, changing the valve overlap in this way fails to acceptably increase the charging efficiency of fresh air under higher or heavier engine loads, lower the content of oxides of nitrogen (NOx) and hydrocarbon (HC) of exhaust gasses under medium engine loads by external exhaust gas recirculation, or improve fuel economy.
The valve timing changing mechanism described above also decreasingly changes the valve overlap in order to increase the output torque of the engine under higher engine speeds. This also makes it difficult to lower the content of oxides of nitrogen (NOx) and hydrocarbon (HC) of exhaust gasses and acceptably improve fuel economy.